The present invention relates to gas and oil seals that are particularly adapted for use in gas springs and in other high pressure hydraulic and pneumatic sealing applications. More specifically, the seal of the present invention advantageously minimizes the leakage of relatively high pressure gas and lubricating oil between an annularly spaced, axially relatively movable cylinder and piston rod, and hence, from the rod end of the cylinder.
The basic structure and theory of operation of gas springs has long been known. Gas springs include a piston and a cylinder, both having a relatively small diameter, usually under two inches. The piston is designed to move relatively within and with respect to the cylinder and is connected with one end of a piston rod that extends out of the normally sealed, rod end of the cylinder. The other end of the piston rod is usually connected with a device upon which the gas spring exerts force when the spring is actuated. The other, closed end of the cylinder is charged with a relatively high pressured gas, generally nitrogen, and usually at a pressure usually between 500-3,000 psi. Lubricating oil is normally also introduced into the closed end of the cylinder at the time of the gas spring's manufacture.
To be commercially acceptable, it has long been recognized by those in the gas spring art that a gas spring must include a seal that minimizes the leakage of the high pressure gas--and in most instances, the lubricating oil--from the rod end of the cylinder. This gas spring seal must effectively seal around the piston rod as well as between the piston rod and the cylinder, or more specifically the inner cylinder wall.
For this purpose, currently available gas springs generally use so-called "lip" seals or so-called "quad" seals. Examples of such quad seals are disclosed in U.S. Pat. Nos. 3,550,990 and 4,693,343. Such quad seals are normally employed in combination with a metal bushing member and a plastic, usually polytetrafluroethane ("PTFE"), washer. The washer is used to prevent the extrusion of the seal as disclosed in the latter patent.
Over the past decade or so, gas springs have been increasingly used in automotive vehicles in lieu of mechanical springs. For example, gas springs are now commonly used to hold open trunk hoods, deck lids, hatch doors (in hatch back type vehicles) and the rear doors or gates of minivans and sport utility vehicles.
The design and manufacture of gas springs for automotive usages pose unique, special problems for gas springs manufacturers, particularly in view of the large numbers of gas springs that must be manufactured to exacting specifications by mass production techniques and machinery. Additionally, and increasingly within the past several years, enormous pressures have been exerted by automotive manufacturers to have gas spring manufacturers reduce their manufacturing costs while, at the same time, enhance the quality of their gas springs.
One of the problems confronting gas spring manufacturers is the range of environmental conditions under which the gas springs are used and their sometimes irregular usage. For instance, gas springs are expected to function satisfactorily in the heat of summer and in the cold of winter even when, for example, a trunk hood may be opened by an elderly person who only uses his or her vehicle infrequently.
To enable gas springs to function as intended in an automotive environment, the gas pressure of the spring (that is, the output force exerted by the spring) must be maintained substantially constant throughout the anticipated life of the gas spring. It is imperative then that leakage of gas from the gas spring cylinder be minimized, both when the gas spring is being used, that is, when it is in a dynamic state or mode, and when it is not being used, that is, when it is static. No one is "happy" when a trunk cover fails to remain in its open position even if the vehicle is over five years old.
Leakage of the lubricating oil from the gas springs cylinders has also been becoming an increasingly serious problem as gas springs are more often used in passenger occupying parts of vehicles, oil "spots" in their vehicles, particularly when the spots are where children or pets are likely to be. Acceptable gas spring seals must now minimize both dynamic and static oil losses.
Only a decade or so ago, the standards for gas springs, set by automotive manufacturers, were losses of less than a 5% output force/gas charge and 3.0 cc. oil per 10,000 cycles. Losses of less than a 5% output force/gas charge and 0.3 cc. oil per 50,000 cycles are now the targeted standards. Automotive manufacturer would like in the future to extend the standards to 100,000 cycles as the life expectancy of automotive vehicles is extended.
Another problem long facing gas spring manufacturers is the reduction of the high static frictional ("stiction") or break-a-way force, that is, the force required to "unstick" the seal when the initial relative movement between the cylinder and the piston rod occurs. This problem is significant in gas springs and hydraulic/pneumatic suspension applications where it is important to minimize high initializing force spikes and resulting seal instability due to long delays between cycles. Such delays are especially common in gas springs employed in automotive vehicles.
Further, dynamic friction may contribute to accelerated seal failures during high frequency low amplitude cycling (typically 3 mm.times.20 Hz). This is associated with "gate dance" which occurs when the vehicle encounters irregular road surface conditions.
Those working in the gas spring art have recognized that currently available gas spring seals, and even the better performing quad seals, have inherent weakness or limitations. For instance, the current, commercially available seals remain prone to relatively high static and dynamic oil losses and has high "stiction" or static break-a-way forces. It has been a longstanding goal in this art to overcome these weaknesses, to extend the cycle life for effective gas spring seals, and to reduce significantly the cost of manufacturing gas springs even further.